Fuel injection systems for compression ignition engines typically include a plurality of fuel injectors, each of which is associated with one cylinder of the engine. It is known to provide each injector with an associated pumping element, in the form of a pumping plunger, which is slidable within a bore provided in a barrel under the influence of a cam arrangement to cause pressurisation of fuel within an associated pump volume or chamber. In a so-called unit injector, the pumping plunger is arranged within a common unit with the associated injector, so that fuel that is pressurised within the pump chamber of the unit is delivered only to the associated injector, and to no other injectors. Multiple-cylinder engines therefore include a multiple number of such units, with one unit being provided for each engine cylinder. As an alternative, unit pumps may be used for which the injector is not housed within the same unit as it's dedicated pumping plunger.
Each unit injector has an associated cam arrangement, each of which includes a lobed cam mounted upon a camshaft upon which the cams associated with units of the same bank of engine cylinders are also mounted. The camshaft may be driven through a gear train by an engine crank shaft, or by means of a belt or chain drive. In a four-stroke engine, the camshaft is driven at half of the speed of the crank shaft. Each unit injector has a drive member that is usually coupled to one end of the plunger through an intermediate member. A cam follower or roller rides over the surface of the associated cam as it is driven, thereby imparting drive to the drive member, and hence to the plunger, to cause the plunger to reciprocate within its bore.
For each complete 360 degree rotation of the camshaft, each pumping plunger performs a pumping cycle including a pumping phase and a retraction phase. During the pumping phase the plunger performs a pumping stroke in which the plunger is driven inwardly within the bore by means of the cam drive, against the force of a return spring. During the pumping stroke the volume of the pump chamber is reduced, causing fuel within the pump chamber to be pressurised to a relatively high level. During the retraction phase of the pumping cycle the plunger performs a return stroke, during which the pumping plunger is urged outwardly from the bore to increase the volume of the pump chamber. The return spring acts in combination with residual fuel pressure within the pump chamber to effect the plunger return stroke. The unit injectors are assembled relative to the camshaft with the lobes of the cams angularly offset from one another, and so that the pumping phase of each plunger is coincident with the end of the compression stroke for the associated cylinder. In a four cylinder engine, for example, having four unit injectors, when one pumping plunger is at the start of its pumping stroke, a second may be at top dwell and a third and a fourth may be part way through their return strokes.
The surface of each cam is profiled to include a rising flank and a falling or trailing flank. When performing the pumping stroke, the cam follower rides up the rising flank as the cam rotates, and during the return stroke the cam follower rides down the falling flank. The cam surface may be profiled such that at the end of plunger pumping stroke but just before the return stroke commences, the cam follower dwells for a period of time at the peak of the rising flank (referred to as “top dwell”). Similarly, at the end of the return stroke, the cam follower dwells for a period of time (referred to as “bottom dwell”) before commencing the pumping stroke of the next pumping cycle. Essentially, therefore, the pumping cycle includes four phases; a pumping phase, a period of top dwell, a retraction phase and a period of bottom dwell.
It has been recognised that in multiple cylinder fuel injection systems of the aforementioned type, the camshaft experiences a variable torque loading throughout the pumping cycle. Variable torque loading arises partly as a result of the cumulative effect of the return spring loads acting on the cams. Such variation between positive and negative torque loading, or “torque reversal”, can give rise to undesirable impacts between the cam and its gear train or other drive components, due to backlash or play between the components.
There is an additional torque loading on the camshaft due to the force of high pressure fuel within the pump chambers. For each unit injector, the torque loading due to fuel pressure is high during the pumping stroke but is much lower for the remainder of the pumping cycle. The resultant, pulsating positive torque loading acts in phase with the positive torque loading that originates from the return springs. As a result, it has been found that the camshaft experiences several large positive torque loading pulses (one for the combustion event within each cylinder) separated by periods of negative torque loading.
Our co-pending European patent application EP 1359 316 A describes a hybrid fuel injection system in which one or more cam driven pumping elements is combined with a common rail accumulator volume to enable injection at two different fuel pressure levels. Due to the provision of the common rail accumulator volume, significant fuel pressures are present within the pump chambers throughout full rotation of the cam, and the camshaft therefore experiences periods of particularly large negative torque loading between the positive torque loading pulses. The problems associated with torque loading reversal, as described previously, are therefore exaggerated in hybrid systems of this type.
It is an object of the present invention to provide a cam arrangement that avoids or alleviates the aforementioned problem. It is a further object of the present invention to provide a fuel pump arrangement incorporating such a cam arrangement.